The subject matter disclosed herein relates to control valves and, in particular, to embodiments of a valve positioner for control valves.
Control valves regulate transmission and distribution of fluids (e.g., liquids and gases). These devices integrate into process control systems in a wide variety of industries. Examples of process control systems form a control loop with remote sensors and other feedback elements to monitor process conditions (e.g., temperature, pressure, etc.). The control loop can generate signals that cause the control valve to modify the flow of fluid in response to changes in the process conditions.
Many control valves integrate valve positioners with digital components (e.g., microprocessors) that can process these signals. These digital components afford the control valve with precise control and functionality. Certain types of digital components can also expand data processing and communication capabilities of the valve positioner. These features can improve the quality, accuracy, and speed of the control valve to respond to changes in the process conditions.
Unfortunately, although digital-based valve positioners are more powerful and accurate than conventional mechanical and/or early digital devices, these types of valve positioners still have reliability issues and can fail. Failures often lead to valve downtime for repair and/or replacement of the defective components. More important, however, is that failures that require maintenance of the control valve can render the process system inoperable for extended periods of time. The resulting downtime can lead to expensive production delays and, possibly, run afoul of regulations set forth by any number of government organizations (e.g., the Environmental Protection Agency (EPA), the Occupational Safety and Health Administration (OSHA), etc.).
Most solutions that address the reliability of digital components provide little relief to shorten, or to avoid, downtime of control valves that utilize digital-based valve positioners. For example, some control valves may integrate a mechanical actuator that can change fluid flow in lieu of the digital components. The mechanical actuator does not operate automatically in this configuration. Rather, maintenance and/or operations personnel must intervene to manually operate the mechanical actuator. Other solutions integrate solenoids with the valve positioner to modulate the flow of fluid through the control valve. However, solenoids provide only binary operation (e.g., on/off), which does not allow finite modulation of flow through the control valve. On the other hand, still other solutions include redundant control valves and/or fluid circuits into the process system. The control system and/or operations personnel can divert flow into these redundant circuits to maintains operation of the process in parallel with repair of the defective control valves. Although effective to remedy potential downtime, these redundant systems still require additional hardware and software that can add significant component cost and complexity to the process line and control system.